1. Field of the Invention
This invention relates to polyorganosiloxane foams. More particularly, this invention relates to one-part, nonslumping foamable polyorganosiloxane compositions that include liquified blowing agents. In the presence of atmospheric moisture and atmospheric pressure, the compositions form foams that cure with a minimum of collapse to yield useful, low density products.
2. Background Information
It is known to prepare foams by introducing a gaseous blowing agent such as compressed air into one-part, moisture curable polyorganosiloxane compositions that are typically employed as room temperature vulcanizable (RTV) sealants. A characteristic of known RTV compositions is the long time period required for these compositions to cure to the extent that the foam becomes self supporting. One way to maintain the cellular structure of the initially produced foam during curing is to maintain a flowable RTV polyorganosiloxane composition under reduced pressure in a vacuum chamber for a period of time sufficient to obtain a self-supporting foam. This technique is taught by Modic and Boudreau in U.S. Pat. No. 4,368,279, which issued on Jan. 11, 1983.
While the application of vacuum during curing may be practical for machine made foams such as slab stock and molded foams, this technique cannot be used when the foam is formed within a cavity of a building or other locations where it is not feasible to maintain the foam under vacuum during curing. In addition, the necessity of mixing in a gaseous blowing agent at the time the foam is formed requires that mixing and aerating equipment be transported to the location where the foam is to be installed. For some applications, particularly those requiring relatively small amounts of foam at remote locations, such equipment would not be practical. In these instances it would be considerably more convenient to employ a one-component foamable composition, including a blowing agent, packaged in a container that can be easily transported to the application site and which is capable of repeatedly dispensing the foamable composition without the need for additional processing steps or ingredients.
A second method for reducing the collapse of partially cured foams prepared using RTV polyorganosiloxane compositions is to incorporate fillers such as silica and calcium carbonate. Filled RTV compositions have been packaged in 2-compartment aerosol cans together with a compressed gas that supplies the pressure required to expel the composition from the can in addition to forming the cellular structure of the foam. One package foamable compositions are described in U.S. Pat. No. 4,229,548, which issued to Sattleger et al. on Oct. 21, 1980, and in German published application No. 2,911,971, which was published on Oct. 9, 1980. The cured foams are typically of relatively poor quality and characterized by cell concentrations of less than 4 per linear centimeter and densities from 0.58 to 0.81 g./cc due to drainage of uncured or partially cured liquid from the cellular structure of the foam during the curing process. The need to minimize collapse and flow of partially cured foams by the use of vacuum, by heating to accelerate curing, by the addition of large amounts of fillers and the resultant increase in foam density or by other means requiring additional material or processing steps may more than offset the advantages achieved by packaging foamable compositions in portable containers such as aerosol cans.
A disadvantage of silica-filled foamable compositions containing an amount of liquified blowing agent sufficient to reduce the density below 0.58 g./cc is that these compositions will flow for a distance of several centimeters when placed on a vertical or sloping surface. This phenomenon is referred to as "slumping."
The present invention is based on the discovery that the slumping resulting from incorporation of liquified blowing agents into prior art RTV foamable polyorganosiloxane compositions can be substantially reduced or eliminated by including in said composition the unique combinations of fillers described hereinafter. The resultant foams exhibit a desirable combination of low density, small average cell size and high cell concentration.